Plastic salt-bridge junctions



March 8, 1960 w. E. PROCTOR, JR

PLASTIC SALT-BRIDGE JUNCTIONS Filed Aug. 29, 1957 Heater Heater PLASTIC SALT-BRIDGE JUNCTIONS Application August 29, 1957, Serial No}; 681,083

7 Claims. (Cl. 204-195)" This invention relates to salt-bridge tubes such asemployed'with reference electrodes, ion-concentration cells or the like and has for an object the forming of a minute passage for the salt-bridge connection in a Zsalt-bridge tube from inert plastic material. It is a further object of this invention to provide a salt-bridge tube with a saltbridge connection formed from a plastic material and of relatively sturdy, construction so as not to be damaged through usage and shaped to resist removal from'the tube.

In accordance with the inventio'n, there is provided a salt-bridge tube having .a 'wall of non-conductive material witha hole therethrough and a plastic structure in the hole and extending through the-, wall. The plastic structure has apair ofintegral flanges, one engaging each of the opposite sides ofthe wall'adjacentthe hole,.the flanges being of suflicient area to retain the plastic structure in the hole inavoidance of accidental displacement therefrom. The plastic structureis formed from a material having a coeflicient of thermal expansion substantially difierent from that of the wall, so that the plastic structure cooperates with the wall to form a minute passageway between the wall and the plastic structureto connect the opposite sides of the wall and provide a saltbridge connection. The plastic structure may be formed from suitable materials, such, for example, as polyethylene, fluorocarbon resins, or the like. i

i For furtherobjects and advantages of the invention and for amore detailed disclosure thereof, reference is to be had to the following description taken in conjunction with the accompanying drawings in which:

Fig. 1 diagrammatically illustrates in section a reference cell incorporating a salt-bridgetube utilizing the present invention;

1td States Pa e as a saturated'KCl solution, in suflicient amount so 2,927,887 Patented Mar. 8, 1960 that . when the electrode 11 is inserted, the solution level will be below the small breather opening 20a. The upper end of tube 20 is threaded so it may be screwed into the cap 14 to seat firmly against a Neoprene washer 15. At the lower end of the tube 20 is a novel salt-bridgecon- I nection formedby a plastic rivet-like structure or plug 22 being measured to a depth so that there is outward flow which extends through an opening 21 in the tube 26, as later to be described more in detail. The lower end of tube. 20 is adapted to be immersed into the solution of solution.

between the salt-bridge solution and the test solution has taken various forms such as wicks, groundjoints, fibers, agar-gel connections, small-bore tubes and various others, all of which are subject to certain serious disadvantages, as well known in the art. Probably the two most frequently used forms, at least in recent years, are the fused glass plug type described in Perley Patent No. 2,345,498 and a metal plug type described in Arthur Patent No. 2,705,220. While both of these types have been used extensively, nevertheless, they are subject to certain disadvantages and limitations. In the fused glass construction there is a resulting crack formed in the glass, and

such crack tends to Weaken the tube at that location. Furthermore, inasmuch as both the tube and plug are formed from glass, they are relatively fragile and caremustbe taken not to dislodge the glass plug from the, end of the tube. This latter precaution must also be taken in connection with the metal plug constructions as they are even more subject to dislocation'than the glass plugs. In addition, the metal plug constructions =are subject to limitation as to many'types of solutions, such, as oxidation-reduction solutions where in such environ-. ments the metal plug causes erroneous voltage readings. Other solutions in which the metal plug type units are not recommended include photographic solutions, cold wave solutions, plating solutions, verystrong' reducing agents, peroxides and strong metal complexing agents.

These foregoing disadvantages have been completely overcome in accordance with the present invention, where the plug in the tube has been co'nstructed from a plastic; material which has the characteristic of being chemically inert with respect to solutions, and in addition has the mechanical strength characteristic whereby it resists dam- Fig. 2 is a sectional 'VlGWOf a supportingtube with a holein the end thereof adapted to receive a plastic: rivet;

Figs. 3 and 4 are diagrammatic views illustrating different methods of forming the plastic rivet in the hole in the supporting tube; and i Fig. 5 is a sectional view, on enlarged scale, showing the plastic rivet in place in the supporting tube; In general, systems of the type to which the present invention is applicable will include'a measuring or pH- responsive electrode and a reference electrode. Examples of such systems are disclosed in Perley Patents "Nos.

tends through a cap member 14. The electrode 11 is disposed in a salt-bridge .tube 20 which has a small breather hole 20a near the-top-of tube 20. The tube20 is .adaptedgto receivea suitable salt bridge solution, such age or-displacement'from the tube after it is inserted.

.One method of making the plastic salt-bridge junctions.-

in accordance with the present invention is disclosed in Fig. 3. The supporting tube 20 is disposed vertically with hole or opening 21 at the lower end. A plastic pellet inert non-conductive material such as glass.

22 is disposed inside the tube 20 and over. the hole 21. The pellet 22 is heated by a suitable source, not shown, to a temperature sutficient'to cause it to flow by gravity into the hole 21. -The temperature to which the plastic is heated is below the melting temperature of the tube 20. While the plastic material is hot, it is manipulated as it flows through the hole 21 to form a rivet-like struc- 1 ture with end flanges 22a and 225 respectively adjacent the inner and outer walls of the tube 20 and as shown in Fig. 5. While the plastic material 22 ishot, itcloses the hole 21 at the end of tube 20 but after it cools, the plastic rivet-like structure produces a minute passage between the latter and the tube byreason of the difierence of the coefficient of expansion or the-plastic material fand' of the material from which the tube is fmade The plastic ma terial has a coefiicient of expansion several'times greater, than the material from which the tube 20 is made; For example, thesupporting tube 20 is usually made j wee as;

plastic material cools, there is formed an opening around the circumference of the plastic rivetin the hole 21 and between the rivet and wall of the supporting tube 20.

Another method for forming a plastic salt-bridge con: nection in accordance with the present invention is illus- For satisfactory results, it was found that the tube 20 or 20 should have a hole 21 or 21 of diameter in the order of inch. This dimension does not change during the formation of the plastic rivet in the tube, as the temperature to which the plastic is subjected during tion will not he accidentally dislodged due to ro gh handling or otherwise. Inasmuch as the plastic material is of higher mechanical strength than the glass supporting tube, the tube will be broken before v the junction is dislodged from the end of the tube.-

D trated in Fig. 4. In this method, a pellet of plastic mathe operation is less than the melting temperature of the terialZZ is heated to its flow temperature and the cold glass. supporting tube 20' is placed over the hot plastic material The following table indicates the temperature and exwith the hole 21' in the tube 24) in alignment with the pansion characteristics of the glass of the supporting tube plastic material 22. By moving the tube 20 down- 10 when formed of a Pyrex glass and also of various types wardly into contact with the plastic material 22, the latof suitable plastic rivets.

Table Material. g g glass i i rex) I i I oi iiifiene 5 1K1 5 (jgfig ientf C,oflines.rexpan- 32 1oten-700x10 1,6001,800 10- ,0001,500 0-- Effect of weak acids none do resistant.

Etiect of strong acids -d0 slow attack by attacked.

oxidizingagcnts.

ter is caused to enter and close the hole 21 to an extent As may be seen from the table, the rivets made from sutficient to form flanges of the plastic material on the Kel-F are extremely inert to chemicals, and alkalies opposite sides of the wall of the tube and around the or acids, whether Weak or strong, have no effect on them. hole. These flanges are illustrated in Fig. 5 by reference The next most suitable material for formation of the characters 22a and 2212. When the rivet-like structure rivet structures was polyethylene. This. material, while formed from the plastic material is cooled, there is provery resistant to the effect of alkalies and weak acids, duced a minute passage 21a around the plastic structure is subject to slow attack by oxidizing agents, and thus and through the hole 21. This passage is of dimension for some applications is less desirable than the preferred suflicient for permitting leakage rates in the order of 1 Kel-F .or other equivalent fluorocarbon resins. to 2 milliliters per day with a liquid head of about 2". What is claimed is:

' The foregoing methods have been found to produce 1. A salt-bridge tube having a wall of non-conductive satisfactory salt-bridge junctions with various types of material with a hole therethrough, said material having plastic materials, such, for example, as polyethylene, properties of chemical inertness, high flow-temperature fluorocarbon resins and nylon. Of the fluorocarbon and coefficient of thermal expansion similar to that of resins, the form identified as polychlorotrifluoroethylene glass, a plastic structure in said hole and extending through and sold under the traemark Kel-F has been found to 40 said wall, said plastic structure having a pair of integral be particularly suitable. flanges one engaging each of the opposite sides of said.

The amount of plastic material utilized in the formawall adjacent said hole, said flanges being of sufiicient tion of the rivet-like structure is in the order of 0.008 area to retain said plastic structure in said hole in avoidcubic centimeter. While the foregoing quantity 'is not ance of accidental displacement therefrom, said plastic critical, it is important that there should be sufficient structure being formed fromamaterial havingacoeflicient plastic material to fill the hole 21 in the tube. Also, to of thermal expansion several times that of the wall so form flanges of the rivet on the inner and outer surthat said plastic structure cooperates with said wall to faces of the tube, it might be expected that the plastic form a minute passageway between said walland said rivetswould completely close and plug the holes in the plastic structure to connect the opposite sides of said supporting tubes when exposed to high operating temwall and provide a saltwbridge connection. peratures. However, this was found not to be the case. 2. A salt-bridge tube having a wall of non-conductive It was found that within a wide working range of from material with a hole therethrough, a plastic structure in about 2 C. to 90 C. the minute passage annulus formed said hole and extending through said wall, said material betweed the plastic rivet and the tube remained open and being chemically inert and having properties of high the junction functioned satisfactorily. The mobilities of flow-tcmperature and low coeflicient of thermal expanthe conducting ions K+ and Cl increased markedly with sion as compared with said plastic structure, said plastic temperature, thus lowering the electrical resistance. structure cooperating with said wall to form a minute Ionic velocities increase about 2 to 2.5 percent for every passageway connecting opposite sides of said wall and degree rise in temperature. This effect opposes the providing a liquid junction, said plastic structure being' diminution of the orifice resulting from the thermal provided with with a flange disposed on the inside of said expansion of the plastic in glass. It was also found that tube to prevent said plastic structure from falling through after the salt-bridge tubes had been subjected to the range said hole, and said plastic structure being provided with Ofttempe'ratures f o 2 c t 90 Q, th re i t la flange at the outer surface of said'tube for preventing ues of the junctions returned to the previously obtained said plastic structure from being forced inwardly of said room temperature values when the temperature was retube. I turned to room temperature or about 25 C. (i5 3. A salt-bridge tube according to claim 2 wherein said As may be seen in Fig. 5, the flanges 132118216 22b serve plastic structure is formed from. polyethylene. to hold theplastic rivet firmly in place inthe end of 4. A salt-bridge tube according to claim 2 wherein the supporting tube 20. As the plastic material from said plastic structure is formed from a fluorocarbon resin.

which the rivet is formed is. of relatively high mechanical 5. A salt-bridge tube according to claim 4 wherein strength, and resistantto breakage, it insures that the junc said plastic structure is formed from polychlorotrifluoroethylene. I

6. A salt bridge tube having a wall of glass with a hole therethrough, a plastic structure in said hole and extending through said wall, said plastic structure having a pair of integral flanges, one engaging each side of said wall adjacent said hole, said flanges being of suflicient area to retain said plastic structure in said hole in avoidance of accidental displacement therefrom, said plastic structure being formed from: a material having a eoeffieient of thermal expansionf several times that of the glass so that said plastic structure cooperates with said glass wall to form a minute passageway between said glasswall and said plastic structure to connect the opposite sides of said wall and prouide a salt-bridge connection.

7. A salt-bridge tube having a wall of glass with a hole therethrough, and for a salt-bridge connection a rivet-like structure of plastic forming a closure for said i of 1 to 2 milliliters per day with a 2" head of solution.

References Citedin the tile of this patent UNITED STATES PATENTS 2,252,504 Hahn Aug. 12, 1941" 10 2,697,070 Arthur' 13%.14, 1954 2,705,220 Arthur Mar. 29, 1955 Schlabach et a1. Apr. 5, 1955 

1. A SALT-BRIDGE TUBE HAVING A WALL OF NON-CONDUCTIVE MATERIAL WITH A HOLE THERETHROUGH, SAID MATERIAL HAVING PROPERTIES OF CHEMICAL INERTNESS, HIGH FLOW-TEMPERATURE AND COEFFCIENT OF THERMAL EXPANSION SIMILAR TO THAT OF GLASS, A PLASTIC STRUCTURE IN SAID HOLE AND EXTENDING THROUGH SAID WALL, SAID PLASTIC STRUCTURE HAVING A PAIR OF INTERGRAL FLANGES ONE ENGAGING EACH OF THE OPPOSITE SIDES OF SAID WALL ADJACENT SAID HOLE, SAID FLANGES BEING OF SUFFICIENT AREA TO RETAIN SAID PLASTIC STRUCTURE IN SAID HOLE IN AVOIDANCE OF ACCIDENTAL DISPLACEMENT THEREFROM, SAID PLASTIC STRUCTURE BEING FORMED FROM A MATERIAL HAVING A COEFFICIENT OF THERMAL EXPANSION SEVERAL TIMES THAT OF THE WALL SO THAT SAID PLASTIC STRUCTURE COOPERATES WITH SAID WALL TO FORM A MINUTE PASSAGEWAY BETWEEN SAID WALL AND SAID PLASTIC STRUCTURE TO CONNECT THE OPPOSITE SIDES OF SAID WALL AND PROVIDE A SALT-BRIDGE CONNECTION. 